I think it's a fine balance between in ring and accomplishment.

1) In ring ability - what did the fighter bring to the table? Were his skills elite-level? Skill/intelligence/physical attributes/ect. What did he do great? What was he not so good at? How did his skills/physical attributes/intelligence stack up with that of other established ATGs?

2) Accomplishment - Was the fighter ever THE guy in his division? How long? Was he ever THE guy in the sport of boxing? For how long? Did he fight a reasonable amount of top quality, top ranked opposition?

These are very subjective obviously, and of course allow human bias to skew them to make a fighter look good or shitty (depending on the bias).

For instance, take Marciano. A lot of people generally say his quality of opposition wasn't as great as other HW champs, that he fought in a weak division. There may be some validity to that, but he is undoubtedly an ATG because he was THE guy in the HW division for several years and fought the top guys of the time. Then there are people who take it the other way, and say "well 49-0 he retired undefeated so that means he's the greatest HW ever". So it's a fine balance.